a. Field of the Invention
The present invention relates to pressure transducers. More specifically, the present invention relates to pressure transducers which convert changes in pressure into changes in the length of a sensing optical fiber and which measure that length change using interferometer measuring techniques.
b. Description of Related Art
Various applications require precise readout of pressure. A principal application is the instrumentation of pressure in oil wells. In many such oil well installations, the pressure sensor transducer must be placed deep in the well where the temperatures and pressures are very high, and replacement of the transducer is an involved and costly process. On-going pressure measurement in an oil well is an important parameter in maximizing its overall oil yield over the life of the well.
One technique that has advantageously been used in remote pressure measurement utilizes fiber optics in the sensor. Changes in the pressure to be measured are converted into mechanical movement. That movement, in turn, causes changes in the length of a sensing optical fiber. The length of a reference optical fiber is then compared to the length of the sensing optical fiber. The length of a reference optical fiber is then compared to the length of the sensing optical fiber using traditional interferometer techniques. Changes in this relative measurement serve as a measure of changes in pressure.
Fiber optics pressure sensors that utilize a single wavelength light source along with traditional interferometer measurement techniques are primarily useful as sensors of pressure change, not the actual pressure value. If the need is to measure pressure accurately as well as pressure change, an ambiguity problem arises. That pressure measurement ambiguity occurs in the interferometer output of the pressure sensing transducer if a single wavelength light source is used. The ambiguity arises whenever the differential length change in the pressure sensor interferometer exceeds one wavelength of light, or a multiple thereof. If the pressure sensor interferometer differential length is constrained by design to be less than one wavelength of light, then the ambiguity is avoided, but pressure measurement accuracy becomes difficult if the pressure range to be sensed is large.
Alternatively, a swept wavelength light source could be used to resolve interferometer ambiguities and thus provide good accuracy over a large pressure range. But present-day solid state lasers suitable for optical coupling into an optical fiber are prone to mode hopping, instead of wavelength sweeping smoothly. Furthermore, the wavelength excursion of the light source must be measured in order to calculate the sensed pressure.
A technique that provides good pressure accuracy over a wide pressure range, while avoiding these measurement ambiguities in the fiber optics interferometer, is taught in U.S. Pat. No. 4,859,844, issued to E. Herman, et al., entitled COMB FILTER PRESSURE/TEMPERATURE SENSING SYSTEM. In that invention, a wideband solid state (infra-red) light source is utilized along with a combination of optical and electronic signal processing in the remote readout apparatus. In that system, by dissecting the optical interference nulls created by the pressure sensor transducer interferometer across a relatively wide optical wavelength spectrum, and by measuring these nulls as a direct function of the optical source wavelength in the readout apparatus, good accuracy of pressure sensing is achieved over a large pressure range. The fiber optics pressure transducer of the present invention is directly applicable for accurately measuring pressure in that sensing system.
Most fiber optics pressure sensor transducers also suffer several other problems. Changes in temperature often result in an indication of a pressure change when, in fact, no pressure change has taken place. Design imperfections in the pressure-to-motion converter often result in hysteresis, and overall non-linearities in the measurement system. Adjusting the tension of the sensing optical fiber and rotating it to optimize polarization are operations which are also often difficult to perform.